1. Field of the Invention
The present invention relates to a hydrogen generator. In particular, the hydrogen generator comprises a water electrolysis cell which includes a proton conductor or membrane, or a plurality of the water electrolysis cells stacked together.
2. Description of the Related Art
Developments are now under way in the field of the hydrogen generation technology which utilizes the conduction phenomena or the ion exchange of cathode ions in polymer electrolytes.
FIG. 10 illustrates a construction of a conventional hydrogen generator which comprises a polymer electrolyte membrane (hereinafter referred to as a "proton conductive membrane"). For example, a proton conductive membrane 1 illustrated in FIG. 10 is made of a fluorocarbon resin having sulfonic acid groups, is provided with electrodes, e.g., platinum 2, 2, on both of the surfaces, and is a non-porous membrane. The conventional hydrogen generator comprises the proton conductive membrane 1 thus constructed, a pair of collectors 3, 4 holding the proton conductive membrane 1 therebetween, an anode terminal plate 5 connected with the collector 3, and a cathode terminal plate 6 connected with the collector 4.
Further, the conventional hydrogen generator is provided with an end plate 7 disposed therearound, and the end plate 7 is provided with a lower groove 8, a first upper groove 9 and a second upper groove 10 formed therein. Accordingly, the water to be subjected to electrolysis is introduced into the conventional hydrogen generator through a water supply port 8a of the lower groove 8, and is supplied to the collector 3 by way of the anode terminal plate 5. The collector 3 consumes some of the water to generate oxygen. The oxygen thus generated is discharged out through the first upper groove 9, again, by way of the anode terminal plate 5 together with the rest of the water.
The proton conductive membrane 1 conducts the hydrogen ions generated at the collector 3 to the cathode terminal plate 6 by way of the collector 4. The hydrogen ions thus conducted are converted into hydrogen gas at the collector 4, and are discharged out through the second upper groove 10 of the cathode terminal plate 6.
The thus constructed conventional hydrogen generator is disposed in a circulation circuit including a water tank. A predetermined voltage is applied between the anode and cathode terminal plates 5 and 6, thereby achieving a water electrolysis process which is affected less by the gas resistances and the ohomic losses. Thus, hydrogen can be generated continuously.
In the conventional hydrogen generator, when a fixed electric current flows in a unit area of the proton conductive membrane 1, the hydrogen generation increases in proportion to the area of the proton conductive membrane 1. However, the enlargement of the area of the proton conductive membrane 1 results in the increment in the overall amount of the electric current flowing in the proton conductive membrane 1, and eventually results in the upsizing of the electric power source. Consequently, the area of the proton conductive membrane 1 and the size of the electric power source are determined depending on the applications.
For instance, in the case of a conventional hydrogen generator to be boarded on a vehicle, the total area of a proton conductive membrane 1 is limited to about 100 cm.sup.2, and the electric current flowing in a unit area of the proton conductive membrane is restricted to fall in a range of from 200 to 300 mA/cm.sup.2. Even if such is the case, an electric current flows in an overall amount of from 20 to 30 A in such a hydrogen generator, and accordingly its battery consumes the electric current excessively.
Hence, a method has been devised in order to reduce the electric current consumed at the battery. In the method, the conventional hydrogen generator comprising the proton conductive membrane 1 is made into a cell, and a plurality of the cells are stacked. With this method, the cells can be connected in series or in parallel, and the area of the proton conductive membrane 1 per one cell can be reduced in each of the cells. Accordingly, the electric current can be consumed less at the battery while keeping a predetermined hydrogen generation. For example, in the case in which five cells are connected in series, the area of the proton conductive membrane 1 per one cell is one fifth of 100 cm.sup.2, e.g., 20 cm.sup.2, the unit voltage to be applied to one cell is from 2 to 2.4 V, the overall voltage to be applied is five times of the unit voltage, e.g., 10 to 12 V, and the electric current is from 4 to 6 A. As a result, the electric current capacity of the battery can be reduced remarkably.
FIG. 11 illustrates such a hydrogen generator comprising five stacked cells. As illustrated in FIG. 11, one of the cells comprises the proton conductive membrane 1, and collectors 3 and 4 disposed respectively on the bottom and top sides of the proton conductive membrane 1. The voltage is applied to the top and bottom cells by way of the terminal plates 5 and 6. Four bipolar electrode plates 11 are disposed between the cells. A manifold 12 adapted to supply the water is piped to each of the anode side collectors 3 on one of the sides (i.e., the right-hand side in the drawing), and a manifold 13 adapted to discharge the oxygen and the water is piped to each of the collectors 3 on the other side (i.e., the left-hand side in the drawing). A manifold 14 adapted to discharge the hydrogen is piped to each of the cathode side collectors 4 on one of the side (i.e., the left-hand side in the drawing).
Through the water supply manifold 12, the water is returned from the water tank (not shown) by way of an ion-exchange resin 15 and a check valve 16. The check valve 16 is operated so as to open and close depending on the pressures of the oxygen gas taken out of the discharge manifold 13. It is thus possible to control the water supply from the water tank to the hydrogen generator.
In the hydrogen generator comprising the five stacked cells, the hydrogen gas generated at the cathode sides of the cells is collected in and taken out of the manifold 14 by way of the cathode side collectors 4. The water to be subjected to electrolysis is divided into and supplied to each of the anode side collectors 3 through the manifold 12, and eventually is collected in and discharged out through the manifold 13 together with the generated oxygen.
However, in the case in which a plurality of the water electrolysis cells are stacked to use, and the generated gases are collected in the manifolds 13 and 14, there occur the differences in the gas generating capability among the cells when the contact resistances or the like fluctuate between the cells and the collectors 3, 4, between the collectors 3, 4 and the bipolar electrode plates 11, and between the bottom or top collector 3 or 4 and the bottom or top terminal plate 5 or 6, and accordingly there might arise a fear for the unbalance in the water supplies to the anode side collectors 3. The fear results from the following phenomena: Namely, the generated oxygen is usually released to the manifold 13, but it tries to go out through the water supply manifold 12. Then, when there arise the differences in the gas generating capability among the cells, the water in the manifold 12 is subjected to the oxygen gas pressure differences. As a result, it becomes hard to supply the water to the cells to which the generated oxygen exerts a large pressure.
Even when there occurs the unbalanced water supply at one of the five cells, the water can be supplied to the cell stack as a whole, as far as the other four cells are in their proper conditions, because the cells are connected by the water supply manifold 12. However, such an operation results in the further deterioration in the water supply to the particular cell, and, in an extreme case, there arises a cell to which no water is supplied at all. Thus, the operation adversely affects the overall hydrogen generating capability of the conventional hydrogen generator considerably.
In order to avoid this adverse operation, each of the cells may be provided with the check valve 16. However, if such is the case, the piping system becomes complicated, and accordingly, such a hydrogen generator is enlarged as a whole so that the advantages associated with the stacked cell construction have been lost.
In addition, the adverse operation resulting from the insufficient water supply occurs even in the conventional hydrogen generator having a single water electrolysis cell construction. Namely, there arises the circumstance that the oxygen is generated so excessively even in the single cell that the water supply cannot catch up with the excessive oxygen generation.